The present invention relates to a process for coating an airfoil to protect it against oxidation and corrosion during operation and to a particular apparatus for use in chemical vapor deposition processes to form an oxidation and corrosion resistant coating on external and internal surfaces of said airfoil.
Aluminide coatings are applied on nickel-based superalloy turbine airfoils to protect the airfoils against oxidation and corrosion during operation in the turbine section of engines. These coatings are formed by the deposition of aluminum onto the surfaces of the airfoils. A reaction occurs between the nickel in the airfoil material and the deposited aluminum to form nickel-based aluminides. At high temperatures, in the presence of air, the aluminum in the nickel aluminide coating forms a thin, adherent aluminum oxide at the surface of the coating. This aluminum oxide provides a barrier against further oxidation and corrosion of the turbine airfoil. The external surfaces of turbine airfoils in most engines are coated with aluminides. The performance requirements of the engines determine whether aluminide coatings also are required on the internal surfaces of the airfoils.
Currently, there are two processes used in the prior art to internally coat airfoils. One process employs a slurry technique and is used for airfoils that operate in the low pressure turbine section of an engine. The second process uses chemical vapor deposition to coat the internal surfaces of the airfoils for the high pressure turbine section of an engine. Different coating processes are employed for the two different types of airfoils because the high pressure turbine section of an engine operates at a higher temperature and pressure than the low pressure turbine section of an engine. As a result, the coating applied to high pressure turbine airfoils must have higher temperature capacity and must be more robust than those applied to low pressure turbine airfoils.
U.S. Pat. No. 5,366,765 to Milaniak et al. describes a slurry technique for coating internal passages in low pressure turbine airfoils. The slurry described in this patent cannot be used to coat the internal passages of high pressure turbine airfoils for the following reasons:
(1) the slurry produces a coating that is too brittle; PA1 (2) the coating is too thick to apply to the internal cooling passages of high pressure turbine airfoils; and PA1 (3) it is not compatible with the processes used to coat the external surfaces of airfoils.
Chemical vapor deposition processes used to coat airfoils employ an apparatus known as a coat boat as generally shown in FIG. 1. During the coating process, turbine airfoils 10 are placed in an upright position within a compartmentalized, large metal box or coating fixture 12, called a coat boat. The figure illustrates a typical coat box arrangement. To coat the internal passages of the airfoils 10, chemicals 14 are placed in a compartment 16 below the airfoils. The airfoils are mounted on specialized plumbing tools 18 that allow vapors to flow through the internal cooling passages of the airfoils. Argon gas is introduced into the lower compartment 16 via inlet 20 to force the coating vapors through the internal areas of the airfoils. These vapors react with the internal surfaces of the airfoil to produce an aluminide coating. At the same, chemicals 14 in an upper compartment 22 create vapors which react with the external surfaces of the airfoil to form an aluminide coating thereon. There are problems however associated with this coat boat process. The problems include (1) the large coat boat which is heavy and ergonomically unsafe to handle, (2) excessive time to coat airfoils due to the large volume of the coat boat, and (3) a fixed capacity due to the fact that the process is a batch process.
Thus, there remains a need for an apparatus and a coating process which eliminates the problems associated with employment of a coat boat in the chemical vapor deposition processes currently employed. There is also a need for an apparatus and a coating process which allows the external and internal surfaces of an airfoil to be coated during a single cycle.